Temporal and vertical distributions of anthropogenic ²³⁶U in the Japan Sea using a coral core and seawater samples

Abstract

The input history of 236U to the surface water of the Japan Sea was reconstructed through measurement of the 236U/238U atom ratio in annual bands of a coral skeleton which was collected at Iki Island in the Tsushima Strait, the main entrance to the Japan Sea. The 236U/238U atom ratios and concentrations of U isotopes were measured for the period 1935–2010 using AMS and ICP‐MS. The 236U/238U atom ratios revealed three prominent peaks: 4.51 × 10−9 in 1955, 6.15 × 10−9 in 1959 and 4.14 × 10−9 in 1963; thereafter the isotope ratios gradually decreased over the next several decades, attaining a value of ca.1.3 × 10−9 for the present day. A simplified depth profile model for 236U in the Japan Sea, using the reconstructed 236U value for the surface water together with observed depth profiles for 236U in the water column in 2010, yielded diffusion coefficients of 3.4–5.6 cm2/s for 6 sampling points. The diffusion coefficient values obtained for the northern stations were relatively large, and fitting uncertainty was also larger for stations in the northern region. It may be presumed that the distribution of 236U in the water columns have been influenced not only by diffusion but also by subduction of the surface water in the Japan Sea.

Multidecadal oceanographic changes in the western Pacific detected through high‐resolution bomb‐derived radiocarbon measurements on corals

“…Considering the ENSO condition in 1954ENSO condition in , 1956ENSO condition in , and 1958, NEC may be strengthened in 1954 and 1956 due to La Niña conditions, which resulted in faster westward 14 C transport than normal condition in 1958. Accordingly, based on a NEC speed of 0.05 m s 21 in 1958, which is minimum estimation of averaged NEC velocity reported by Qiu and Joyce [1992], we determined a KC speed of 0.21 m s 21 , which is consistent with the uranium-based low-resolution estimate reported previously [Sakaguchi et al, 2016]. [Konishi et al, 1981], Guam [Andrews et al, 2016], Palau [Glynn et al, 2013], and Langkai [Fallon and Guilderson, 2008] Site Figure 2 reached a maximum in the 1970s with high levels persisting until the 1980s.…”

“…Considering the ENSO condition in 1954, 1956, and 1958, NEC may be strengthened in 1954 and 1956 due to La Niña conditions, which resulted in faster westward 14 C transport than normal condition in 1958. Accordingly, based on a NEC speed of ∼0.05 m s −1 in 1958, which is minimum estimation of averaged NEC velocity reported by Qiu and Joyce [], we determined a KC speed of 0.21 m s −1 , which is consistent with the uranium‐based low‐resolution estimate reported previously [ Sakaguchi et al ., ].…”

“…Andrews et al [2016] estimated the NEC speed between the US Pacific Proving Grounds site and Guam as 0.1-0.2 m s 21 . The KC speed is known to be much faster than that of the NEC, at approximately 0.5-1 m s 21 , yet Sakaguchi et al [2016] estimated an average speed 0.2 m s 21 for the KC using 236 U/ 238 U in coral collected from Iki Island, Japan. The speed was deduced from the distance between the US Pacific Proving Grounds sites and Iki Island considering also the time differences between the date of the nuclear bomb tests and 236 U/ 238 U signal appeared in the Iki coral.…”

“…The speed was deduced from the distance between the US Pacific Proving Grounds sites and Iki Island considering also the time differences between the date of the nuclear bomb tests and 236 U/ 238 U signal appeared in the Iki coral. Due to the large quantities of samples required for 236 U measurements (e.g., 0.3-0.5 g of coral skeleton) [Sakaguchi et al, 2016], results from this type of calculation have lower resolutions than 14 C-based studies. The averaged velocity of NEC along 1378E varies from 0.05 to 0.2 m s 21 in the area from 108N to 138N for the period from 1967 to 1988 above the depth of 250 m [Qiu and Joyce, 1992].…”

233U/236U signature allows to distinguish environmental emissions of civil nuclear industry from weapons fallout

“…T he long-lived uranium (U) isotope 236 U (T 1=2 = 2.342Á10 7 years 1 ) is increasingly adopted as environmental tracer with advantageous chemical properties especially for oceanography [2][3][4][5] . U shows a conservative behavior in sea water and therefore can be transported over long distances in this environment.…”